[SI-LIST] Re: MGB/s pair crossing on adajent layers

  • From: wolfgang.maichen@xxxxxxxxxxxx
  • To: Hirshtal Itzhak <ihirshtal@xxxxxxxxxx>
  • Date: Wed, 2 Jun 2010 09:23:33 -0700

Hello Itzhak,
as long as the traces are truly perpendicular crosstalk will be minimal. 
The issue then is mostly the restricted routing flexibility - e.g. you 
can't just have the traces make turns at arbitrary locations, lest they do 
run parallel to some trace on the adjacent layer. With fast rise times, 
even short sections of parallel traces will result in full-blown near-end 
crosstalk (NEXT).

To get an initial idea how strong the coupling is for a given length of 
trace, there are a few free(!) tools that at least allow first-order 
estimates, good to build some confidence in your design decisions:

TNT/MMTL field solver (available from Sourceforge) - a 2D solver that you 
can use to get coupling (crosstalk) parameters between parallel traces. It 
even gives you total NEXT and FEXT directly, for given coupling length and 
signal rise time.

Sonnet Lite - the free version supports 2 signal layers, plus you can use 
top and bottom box cover as your ground planes, which allows simulation of 
your 4-layer stack (G-S-S-G). Since it is a planar (2.5D) field solver you 
can use it to simulate more advanced situations, e.g. estimate residual 
coupling of traces running at e.g. 45 degree angles.

(I'm not connected with Sonnet in any way, other than that I am using 
their free software version).

Hope that helps

Wolfgang








Hirshtal Itzhak <ihirshtal@xxxxxxxxxx> 
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06/02/2010 04:37 AM

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[SI-LIST] MGB/s pair crossing on adajent layers






Hi Experts!
 

I use double-stripline layers for MGb (PCI-Ex, sRIO) pair routing. I
ordered the PCB Designer not to route such pairs on adjacent layers,
close to each other, not even PERPENDICULAR to each other.

 

Now it seems I can't avoid it any more. My PCB Designer seems to run out
of real-estate.

 

I have 4-mil separation between the 2 layers and 4-mil from each one to
a GND layer on each side.

 

The pairs are running at 2.5 to 3.125 Gb/s

 

What risks, if any, I run into, if I do permit routing the pairs
perpendicular to each other? Are there any precautions I should take
while doing it?

 

Thanks

 

Itzhak Hirshtal

 

Elta



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